Many ionic polymer membranes used in electrochemical cells are an electrolyte comprising only one active material, having homogeneous properties throughout. The properties of the membrane will have an effect on the performance of the electrochemical cell, such as cross-over coefficient, conductivity, mechanical strength, softness, water content and hydration control. Optimisation of one property may lead to a reduction in performance, due to effects on other properties.
Nafion is a fluorinated polymer with side-chains containing strongly ionic groups. It is a thermoplastic material which shows high conductivity and low cross-over, but suffers from dehydration when used in a fuel cell unless gases and their associated hydration levels are carefully managed. In addition, contact between this material and a catalyst in a MEA requires hot-pressing, and this contact often suffers delamination when the MEA is re-hydrated in or before use.
WO2006/131758 describes graft copolymers that can be used as membrane materials. They are obtained by forming a polymer film, and reacting the film with a material having strongly ionic groups, to form a polymer film having the ionic groups grafted thereon.
WO03/023890 discloses hydrophilic materials including a strongly ionic group, suitable for use in electrochemical cells. They comprise a polymer, optionally cross-linked, obtainable by copolymerisation of hydrophilic and hydrophobic monomers, a monomer including a strongly ionic group, and water.
WO2005/124893 and PCT/GB2007/000949 describe composite membranes utilising hydrophilic cross-linked polymers of the type described in WO03/023890.
WO2006/032887 describes a method of performing an electrochemical reaction in an electrochemical cell comprising electrodes separated by a membrane capable of taking up an electrolyte. The method comprises introducing into the cell an oxidisable component and an electrolyte, and oxidising the fuel in the presence of an acid or alkali.